The present invention relates to high performance tyres such as, for example, tyres designed for high-powered cars or, more generally, tyres intended for applications involving high operating speeds.
These tyres, which are commonly referred to as xe2x80x9cHPxe2x80x9d and xe2x80x9cUHPxe2x80x9d (High Performance and Ultra High Performance) tyres, are in particular those belonging to the Classes xe2x80x9cVxe2x80x9d and xe2x80x9cZxe2x80x9d that respectively provide for maximum speeds over 210 Km/h and 240 Km/h, for which the operating performance is undoubtedly one of the most important features.
This performance depends both on the friction coefficient (or grip) and on the response to lateral stresses of the tyre during rolling on the road surface.
Usually the friction coefficient depends on the properties of the compound used for the tread, while the response to lateral stresses and to centrifugal forces depends on the tyre structure; for these reasons there exist structures designed to compensate for the force systems to which HP and UHP tyres are subject.
One of these structures envisages the presence of a so-called xe2x80x9c0xc2x0xe2x80x9d layer, namely a layer of rubber reinforced by a cord which is usually heat-shrinkable, for example made of nylon, wound spirally in the circumferential direction around the tyre belt.
The winding angle of the cord with respect to a median (or equatorial) plane of the tyre is fairly small and it is for this reason that the layer in which it is located, is referred to as a xe2x80x9c0xc2x0xe2x80x9d layer; the tread band is then mounted on top of this layer.
From Canadian patent CA 1228282 in the name of Bridgestone and European patent EP 592218 in the name of Sumitomo it is also known to form the tread band with what is commonly known as xe2x80x9ccap and basexe2x80x9d structure, namely consisting of two layers circumferentially arranged one upon the other wherein the radially innermost layer forms the xe2x80x9cbasexe2x80x9d or underlayer, while the other outermost forms the xe2x80x9ccapxe2x80x9d and is the layer which is intended to roll on the road.
In particular, the first of these documents suggests to combine an underlayer with an external cap having a high grip and a predefined ratio between its modulus of elasticity and that of the underlayer, in order to improve the steering performance of the tyre at high speeds.
European patent EP 592218 teaches, on the other hand, to add reinforcing fibres to the compound of the underlayer and orienting them in a particular manner so as to obtain an anisotropic behaviour thereof with a different modulus of elasticity, respectively, in a circumferential and a transverse direction with respect to the tyre.
In this way it is possible to obtain a tyre with excellent stability during bends, a high level of comfort during travel as well as a low rolling resistance.
Finally, from European Patent Application EP 691218 published in 1996 it is known to manufacture tyres with properties which are substantially equivalent to those of tyres with a reinforcing layer consisting of nylon cords arranged at 0xc2x0, by providing, in place of the latter, a tread underlayer reinforced by fibres of a particular size and strength.
These fibres consist of materials such as polyamides (in particular aromatic polyamides shortly referred to as xe2x80x9caramidesxe2x80x9d), polyesters or polyolefins, which are already known in the art for their use in the manufacture of tyres.
For example, with regard to aramide fibres, a detailed description of their use in the tyre sector is given in the American patent U.S. Pat. No. 4,871,004, which is hereby incorporated by reference.
One of the reasons justifying the use of the abovementioned fibres lies in the fact that they generally allow to achieve a good structural resistance with a reduced weight.
There are, however, certain technical aspects which must be taken into account when using the fibres in question.
Indeed, their use instead of, or in combination with, materials of the traditional type used in the manufacture of tyres, is an area which today is still not fully known: it is therefore important to be able to optimize the use of the fibres in order to obtain the required features of the tyres.
The HP and UHP tyres currently known are not fully satisfactory from the point of view of their performance at high speed and over long operating periods: the present invention proposes to solve this problem by providing a high performance tyre having structural and operating features that remain substantially unvaried during the different conditions of use.
The invention arises from the Applicant""s realization that the characteristics of a high modulus and/or hardness of the compound of the underlayer, as an alternative to or in combination with each other, must also be guaranteed at the high speeds referred to above and in particular must not deteriorate with an increase in the temperature due to prolonged use at high speed, and that this object may be attained using the abovementioned fibres.
In particular the Applicant has discovered that the problem may be solved using a tyre wherein on the reinforcing layer with cord at 0xc2x0, there is applied a tread band with a xe2x80x9ccap and basexe2x80x9d structure whose underlayer has hardness and/or elasticity values which are substantially stable between 23xc2x0 C. and 100xc2x0 C.
Owing to these features, in fact, the tyre ensures a uniform performance also at the high operating speeds which may cause a substantial increase in the temperature of the tread.
In accordance with one of its more specific aspects, the invention relates to a tyre wherein the hardness of the tread underlayer between 23xc2x0 C. e 100xc2x0 C. does not vary by more than five units on the IRHD (International Rubber Hardness Degrees) scale; preferably this variation must not exceed three units and, even better, one IRHD unit.
According to another more specific aspect of the invention, the tyre tread band has an underlayer with a dynamic modulus (Exe2x80x2) which between 70xc2x0 C. and 100xc2x0 C. does not vary by more than 10% and, preferably, varies by less than 5%.
In accordance with a preferred embodiment of the invention, the tread underlayer also has high ratios (greater than 4) between the rigidity in the direction of travel (i.e. in the circumferential direction) and in the direction perpendicular to it; this means that it has an anisotropic behaviour.
The properties of the tread underlayer indicated above may be obtained using a compound reinforced with a quantity of fibres preferably ranging between 3 and 10 phr (parts per hundred of rubber), and even more preferably between 6 and 9 phr, in combination with hardening resins; the latter are preferably based on resorcinol and methylene donors.
These resins may be either in the form of two components or in the precondensed form, while preferred methylene donors include hexamethoxymethyhuelamine (HMMM) or hexamethylenetetraxnine (HMT); the Applicant has found, however, that other methylene donors and other types of hardening resins may be used.